An ammeter of resistance 5 reads up to 1 A. The value of the shunt re…

The correct answer is Option (2) → 1.25 Ω

Let the resistance of the ammeter be $R_a = 5\ \Omega$.

The maximum current the ammeter can read is $I_a = 1\ \text{A}$.

We want to extend the range to $I = 5\ \text{A}$, so the remaining $I - I_a = 4\ \text{A}$ must pass through the shunt $S$.

Since the ammeter and shunt are in parallel, the voltage across both must be the same:

$V_a = V_s$

$I_a \cdot R_a = (I - I_a) \cdot S$

Substitute values:

$1 \cdot 5 = 4 \cdot S$

$5 = 4S$

$S = \frac{5}{4} = 1.25\ \Omega$